This invention relates to rear projection systems, and more particularly to rear projection systems that fold optics to reduce the cabinet depth of the system.
Rear projection imaging systems typically include an image generation source, optics to enlarge and direct the image and a transmission screen for displaying the enlarged image. The image source can be of many different types, including cathode-ray tubes and LCD projectors. In simple systems, the optics generally includes a lens, such as a combined convex glass lens element and a methacrylic resin lens element, and a turning mirror for directing the image toward the screen. The transmission screens of typical systems generally include diffusing material, lenticular lens sheets and Fresnel lens sheets, which are intended to project a wide image with uniform brightness.
In operation, the image source is positioned behind the transmission screen and provides a small, bright image to the projecting lens. The projecting lens enlarges the image and directs it to the reflective surface of the turning mirror. The turning mirror reflects the image to the transmission screen. The lens sheets in the transmission screen further enlarge the image and collimate the projected light. The audience views the projected image from the transmission screen.
The depth dimension of known rear projection systems is constrained by the angle of incidence on and within the transmission screen""s Fresnel lens. To make a compact rear projection package, a short focal length lens is required. A decreasing focal length increases the field of view as measured at the screen. As the field of view increases, the angles of incidence in air and within the Fresnel lens eventually approach the critical angle, causing transmission to drop to zero.
Even before the angle of incidence reaches the critical angle, the angle of incidence exceeds the Brewster angle. Exceeding the Brewster angle can cause the S (perpendicular) and P (parallel) polarization transmission coefficients to diverge, differing by as much as 50%. A divergence in polarization transmission coefficients results in image distortions, such as non-uniformity in brightness across the screen.
A projection system according to the principles of the invention achieves a reduction in the depth dimension without sacrificing image sharpness or brightness uniformity across the screen. In one aspect of the invention, a rear projection imaging system includes a projector source operable to anamorphically project an image. Anamorphic projection outputs different magnifications along mutually perpendicular radii. The image is projected along a projection path onto a non-planar (curved in a side view) turning mirror. The curved turning mirror directs the projected image to a light collimator, such as a transmission screen. The transmission screen can include a Fresnel lens. The curved turning mirror permits for a decreased cabinet depth; it folds the optical in less space than a planar mirror. Distortions introduced by the non-planar turning mirror correct the anamorphicallly projected image. The anamorphic projection optics and the curved turning mirror both impose respective Scheimpflug conditions on the image, resulting in a sharp image in the image plane.